Is anyone using evacuated tube manifolds for DHW or whole house heating? Looking for some direct experiences.
Evacuate tube solar
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I'd also be interested in this. I've considered building some copper tube collectors and placing them in a box with a double pane window, but evacuated tubes would be nice for better year round collecting.
GaryGary
Feeling the Heat
Hi,
There are a couple of reports on actual installations and side by side comparisons of evacs and flat plates here:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Evacuated Tube
The SRCC site has test results and estimates of output for all of the collectors (flat and evac) that are SRCC certified, which is most of them since you don't get the federal rebate without the SRCC cert.
http://www.solar-rating.org/ratings/index.html
You can compare various brands/models.
One thing I've noticed about the evac tubes is that they vary quite a bit in performance from brand to brand.
Gary
DBoon
Minister of Fire
I have no personal experience with evacuated tube collectors, but have heard that their higher inherent insulation value means they will not shed snow very quickly. This may be more of an issue in areas where there are frequent Lake Effect snow events.
jebatty
Minister of Fire
My experience is limited, and I think a very good design is essential: regardless of what you are told, overheat load dumping and freeze protection likely are essential. Do lots of homework if you try to do this on your own, and if a contractor search out the contractor well for lots of experience, check references on installations, etc. If I made a move towards solar hot water, I think I would head in the flat plate collector direction.
I've noted that, as well. I've seen claims ranging from 200-500btuh/tube. Similar designs will vary based on materials/construction quality but not by 100%+.
Thanks for the links.
That does seem to be the case. I'm not opposed to cleaning them off once the lake stops dumping on us. It's not that frequent an occurance. If I had to do it daily I suppose it would become tedious.
I'm thinking I'll get one assy. before going for a full array. This way, I can see what real world performance is on the less than optimal days. You know, the kinds of conditions the suppliers never want to talk about.
GaryGary
Feeling the Heat
One thing to bear in mind on snow cleanup for the evac tubes is that they are fairly delicate and easy to break, so brushing light snow off them would not be a problem, but getting frozen snow or ice off them without breaking them might be difficult -- but, I have no actual experience.
I did mange to break one of a friends evac tubes just bumping it against a wood support -- luckily he had some spares.
Gary
Much the same as flat plate collectors. Got to treat them well if you don't want to do any damage. I'm just intrigued by the improved performance in winter over flat plates. I just hate seeing the sun shining and knowing I could be getting some heat from it.
I don't have direct experiences but run into folks on occasion trying to find replacement tubes, apparently there were several brands with different types of end connections that are not interchangable. Some designs are better than others and apparently some varieties are losing their vacuum faster than expected. Most of the installs in my area have issues with snow buildup.
After I looked up how many hours of clear daylight I got in the winter using PVWatts, versus the other 9 mos, I got a lot less interested in wintertime solar heating, and then panels started to look a lot better.
I hear you. One supplier had just sold a system for DHW only to a customer very near where I work. I inquired if they would pass along my # so I could talk with the user about their experiences with the tubes. Apparently, the individual indicated they've no desire to be available to curious shoppers. FWIW, said user professed satisfaction with the system so far as it's winter performance was concerned.
I don't doubt their higher eff in the cold, just that there are a lot of BTUs to be had in January in the East coast or Midwest. Mountain states are another story IMO. When you crunch the numbers PV looks great (all year round), solar thermal can be ok if low upfront cost, 9 mos a year, but solar active space heating, I think would be mucho $$$/BTU in OH.
Seems the efficiency advantage oner flat plate is marginal at best real world.
My motivation is not just heat but I wanted the best year round system I could get
Guess I'll just go back to building the flap plate collectors as Iooriginally planned.
My motivation is not just heat but I wanted the best year round system I could get
Guess I'll just go back to building the flap plate collectors as Iooriginally planned.
Most folks are coming to the realization that they are better off buying a few more solar electric panels and a mini split. Its about the same front end cost for installation and with net metering they get a lot more heat when they want it. Definitely not an option for off gird folks.
It all come down to the fundamental problem is that there is way to much heat in the summer and not enough in the winter and no way to store it. PV and net metering is not very efficient but it works pretty well even up in NH.
It all come down to the fundamental problem is that there is way to much heat in the summer and not enough in the winter and no way to store it. PV and net metering is not very efficient but it works pretty well even up in NH.
I intend to add a grid tied PV system. Just in the deciding what size phase, atm. My interest in solar water heat is primarily to heat a pool. If i was able to add btu's to my winter storage, so much the better. I'm sure I'll get some output from flat plate collectors in the winter but I want to be sure I was getting the best bang for my buck.
Since I can fab the flat plate collectors, those will likely brovide the best yield for my investment. Loss of credit will diminish that a bit, however.
Since I can fab the flat plate collectors, those will likely brovide the best yield for my investment. Loss of credit will diminish that a bit, however.
GaryGary
Feeling the Heat
I've been working on a method that would allow people to estimate how much collector area is needed for a solar water heating system that will get an 80% solar fraction for various US climates. I'm using the free retscreen simulation software to do this. It takes into account climate, hot water demand, hot water temp, collector orientation, ground water temp, storage size and a few other things.
For my effort to simplify things, I'm assuming: 48 gallons/day demand, collector pointed south and tilted at the cities latitude, 120F hot water, cities ground water temp, 2.5 gallons storage per sf of collector, typical flat plat collector efficiency curve.
My aim is to put this up on my website when its done, but since it comes up in this discussion, thought I would pass along some results so far.
This is collector area required for an 80% solar fraction for 15 cities given the assumptions above:
Pheonix 22 sf
Miami 31 sf
LA 34 sf
Denver 37 sf
San Fran 40 sf
Salt Lake 48 sf
Topeka 48 sf
Boston 57 sf
Madison 60 sf
Bozeman, MT 68 sf
Bangor 78 sf
Buffalo 80 sf
Portland, OR 100 sf
Seattle 125 sf
Anchorage 195 sf
I would not take these as gospel, but retscreen is used by a very wide audience and they do validate their results against other simulations.
One interesting thing is that the areas vary over such a wide range.
In the end, the stuff I'm planing to put up covers things like cost (DIY), payback, IRR, solar fraction, carbon emissions, ...
If you look at the data above, you might think that Pheonix would be the hands down winner on payback, but several of the cities do better, with Denver the best (2.9 years). A good part of the reason for this appears to be the high ground water temp in the south -- it means that there is not as much money to be saved by heating it with solar, so you save a bit on system cost with the smaller collector and storage, but this is more than offset by the smaller savings -- did not expect that.
Just looking at the numbers, the east coast locations don't look that bad to me. I picked Buffalo because it has reputation for very cloudy winters, and its really not a whole lot worse than Bozeman (where I am).
Anyway, thought this might be of interest.
Any thoughts on the method would be appreciated.
By the way, you can download retscreen and run it yourself if you want to see other conditions -- it also can do solar heating systems. There is a bit of a learning curve, but not awful. http://www.retscreen.net/ its based in CA, but used worldwide -- some areas require a retscreen analysis to get rebates.
Gary
GaryGary
Feeling the Heat
Most of the commercial flat plates are made with tempered glass that is very tough.
Most of the DIY ones are made with either corrugated polycarbonate or twinwall polycarbonate -- almost indestructible.
I was careless with one of the twinwall panels on my large collector and left it only loosely fastened because I was going to do some work on it. We had a major wind storm, and the next day, that panel was missing. I found it a quarter mile away in my neighbors field. It had a few scratches and one abraded corner, but otherwise fine -- I put it back up and I'm still using it years later.
Gary
Thanks for that, Gary. I'll check out retscreen this weekend.
I'm not horribly concerned with a quick payback. I'm not going to be going anywhere. So provided whatever I install has limited maintenance costs and a decent lifespan, I'll be happy.
I intend to start with 256 sq ft of collectors. As I said, my primary goal is pool heating with DHW thrown in for fun. Any performance beyond that is a plus. My array will be mounted to my garage wall and equipped to tilt to track latitude changes through the seasons.
I'm not horribly concerned with a quick payback. I'm not going to be going anywhere. So provided whatever I install has limited maintenance costs and a decent lifespan, I'll be happy.
I intend to start with 256 sq ft of collectors. As I said, my primary goal is pool heating with DHW thrown in for fun. Any performance beyond that is a plus. My array will be mounted to my garage wall and equipped to tilt to track latitude changes through the seasons.
You might consider building a separate system for heating your pool. You could a bigger/cheaper/dumber array that just circulates pool water w/o any HXs, and maybe without any real glazing....there are also low cost solutions you can buy. I would save the nice, durable, well glazed, freeze safe panels for DHW, and those could then be sized a lot smaller/easier/cheaper, and matched to a smaller tank.
solarstar
Member
solarstar
Member
They were fine up to that point . In fact I was quite impressed with performance as a whole , but broken tubes has left me in a pinch. I pretty have have to decommission or replace half my bank
I wondered how susceptable they might be in that case. It's quite certain that we'd see several good hail storms in the course of their lifespan. Apparently that would entail tube replacement to some extent each time.
solarstar
Member
they are Thermomax . out of England .Millions of tubes in service all over the world . Supposed to take a 1" hailstone at 50 MPH . Im convined it was very close to that based on other damage around here that day . tore up vegetation as if a Bush Hog had gone over it . I think a quality Flat plate with tempered glass of course will outlast our lifespan.. as a collector . I will find a picture shortly
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